Is Protein Combining Necessary After All?
Combining complementary plant proteins was fashionable back in the 70s, dismissed in the 90s, and may now be making a comeback thanks to new research. Protein researcher Doug Paddon Jones joins me to discuss his latest study.
Monica Reinagel, MS, LD/N, CNS
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Is Protein Combining Necessary After All?
One of the biggest nutrition trends right now is a move toward more plant-based or plant-forward diets. Although the number of strict vegetarians or vegans has not changed all that much, a lot of people are trying consume fewer animal products. I’m sure you’ve seen the explosion of plant-based milk and meat substitutes in your local grocery stores and restaurants, and we’ve talked about many of them on the podcast. Â
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At the same time, there’s been a lot of interest in how dietary protein affects things like satiety, weight loss, muscle building, aging, and recovery.  We’ve talked about a lot of that research on the podcast as well. In particular, protein’s role in building and preserving lean muscle may be particularly relevant for those who are older, recovering from illness or injury, or losing weight—because these circumstances carry an increased risk of muscle loss.
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The collision of these two nutrition trends—plants and protein—has resurrected an old question: Is it necessary to combine complementary plant proteins at the same meal in order to create a “complete” protein?  Is the traditional combination of rice and beans actually grounded in nutritional necessity?
Protein’s role in building and preserving lean muscle may be particularly relevant for those who are older, recovering from illness or injury, or losing weight.
Joining me to unpack what we do and don’t know about this issue is protein researcher Douglas Paddon Jones, who runs the Nutrition and Metabolism Laboratory at University of Texas Medical Branch in Galveston, Texas.
Following are highlights of our conversation. To hear the entire interview, please click on the audio player above.
Nutrition Diva: If you are a long time listener of the Nutrition Diva podcast, then you are already quite familiar with Doug’s research because it has been the subject of at least half a dozen previous episodes. Â
It was Doug’s research, for example, that I discussed in episode #398, on how to slow age-related muscle loss and again in episode #448 on how to build more muscle with less protein. If you have been eating more protein at breakfast (and maybe a bit less at dinner), or you’ve been shooting for 25-30g of protein per meal, it’s probably because of Doug’s research.
A lot of this research was done using animal proteins, such as beef, or eggs, or whey—because animal protein tends to be a higher quality source of protein. This is not a value judgment against vegetarians, just a biochemical reality.
Doug Paddon-Jones: Very true! To turn on protein synthesis—which essentially means the “muscle growth and repair machinery”—you need ALL the essential amino acids to be available in sufficient quantity at the same time.
You can absolutely get all the essential amino acids you need from plants, but it’s hard to get them all in ONE plant—at least in the proportions that make them most efficient.
Legumes, for example, tend to be lacking in methionine. Grains, on the other hand, are a decent source of methionine but tend to be low in lysine.
That means that, if you eat either of these foods by themselves, you still may be lacking one or more of the key ingredients (amino acids) that are needed to kick start the muscle building machinery.
ND: It’s a little like baking: Let’s say you have enough for flour for four loaves of bread but only enough yeast for two loaves. You’re going to end up with two loaves of bread plus a bunch of leftover flour.
And that’s where the idea of protein combining came in. By making sure to eat both legumes and grains (or some other complementary pair of plant proteins) at every meal, you’d be able to use more of the amino acids to synthesize proteins and not have as many left over.
DPJ: This notion was popularized back in the 1970s, another era when vegetarian diets were trending. But then in the 90s, the nutrition establishment—such as it is—stepped in to say that this was simply folklore. As recently as 2016, the Academy of Nutrition Dietetics, in a position paper on vegetarian diets states that protein combining is unnecessary. The idea is that the body will store unused amino acids in the liver or elsewhere for future use.
ND: To return to our baking analogy, if our baker is left with a bunch of unused flour, he simply puts it in the larder. Assuming the next grocery delivery contains some yeast, he’ll get that flour back out and make the other two loaves of bread. Any leftover yeast will be stored until more flour comes in. And so on.
But what is the evidence to support that this is the way things work?
DPJ: If you do an internet search for “complementary protein” or “protein combining,” most of the top results dismiss the idea that it’s necessary to consume complementary protein at each meal. A lot of these sources cite a 1994 review paper written by Young and Pellett. However, their conclusion (that combining complementary proteins is unnecessary) was not based on a specific study but was essentially their opinion, based on their interpretation of prior research. Since their review was published, we have a lot of new information that was simply not available then.
When it comes to muscle protein synthesis, the body isn’t really going to use amino acids that might be left over from previous meals to fill in any gaps from the current meal.
For example, studies done by my colleagues have shown that it’s not simply the presence of amino acids in the body that triggers protein synthesis. It’s a sudden increase in the amount of amino acids in the blood that kicks the body into protein building mode.
ND: Once more to our baking analogy, the baker doesn’t just hang around the kitchen all day. When supplies are delivered, he goes to the kitchen, unpacks the groceries, and starts baking.
DPJ: And here’s another interesting piece of evidence that seems to undermine the idea that protein combining isn’t necessary: A sudden increase in amino acid concentrations in the blood will trigger a burst of muscle building activity (as long as all the necessary aminos are present in sufficient amounts.) But after a couple of hours, this burst of activity will subside, even if there are still plenty of amino acids lying around.
ND: So regardless of how big the grocery delivery is—or even if groceries are delivered continuously—after an hour or two, our baker is going to turn off the oven and go read the paper until the next shift starts.
DPJ: All of this suggests that when it comes to muscle protein synthesis, the body isn’t really going to use amino acids that might be left over from previous meals to fill in any gaps from the current meal.
ND: In other words, if a grocery delivery contains enough flour for four loaves but only enough yeast for two loaves, the leftover flour doesn’t go into the larder until the next load of groceries arrives. It goes into the compost. It’s not exactly in the trash…it’s still providing some benefit. But it’s not available for future bread baking.
DPJ: Our hypothesis—which we are going to be testing in a randomized, cross-over study—is that it’s not enough to get all of the essential amino acids over the course of a day. In order to maximize protein synthesis, you really do want to get the right proportion of essential amino acids at each meal.
To test this, we’re recruiting healthy men and women between the age of 45 and 60 and we’re going to test the effects of 4 different meal plans. All the meals will contain the same number of calories and the same amount of protein—30 grams per meal. One meal plan will provide 30 g of high quality animal protein (beef) at each meal. One meal plan will provide 30 g of complementary plant-based proteins (beans + grains) at each meal. A third meal plan will provide the same amount of plant-based protein but with all the beans at one meal and all the grains at another. And the last meal plan will be a very low protein diet, with less than 5 grams of protein per meal. For each meal plan we’re going to test its ability to stimulate muscle protein synthesis.
ND: What do you think you’ll see?
Those who choose to avoid animal proteins altogether will pay a price in terms of muscle protein synthesis.
DPJ: We expect that, in terms of muscle protein synthesis, the meal plan where the complementary proteins are separated into different meals will not be any better than the very low protein diet. We think that the complementary plant-protein meals will do quite a bit better but not quite as well as the meal containing the same amount of animal protein.
ND: Why would the complementary plant-based protein meal plan not do as well as the animal protein meal, if both are providing an equivalent amount of protein and essential amino acids?
DPJ: Protein quality is affected not just by the presence and balance of amino acids but also the digestibility of the protein source. Plant-based proteins, especially when delivered as whole foods, are simply less digestible than animal based proteins.
ND: Finally, we’ll have a definitive answer to the question of whether protein combining offers any benefits. But aside from scientific curiosity, what are the real world applications for this research? How does answering this question help us make us healthier food choices?
DPJ: Well, our broad objective is to demonstrate that meals containing a high quality source of protein, (like beef, fish, eggs, dairy etc) are an effective and efficient way to augment a largely plant-based diet and stimulate muscle protein synthesis.
We would all benefit from consuming more fresh and minimally processed plant-based foods, so we’re definitely not trying to demonstrate that plant-based foods or diets are unhealthy or impractical but we do want to challenge the prevalent notion that getting all the essential amino acids over the course of the day is equivalent to getting them at each meal, especially when maintaining lean muscle has so many important functional benefits.
ND: Those who choose to avoid animal proteins altogether will pay a price in terms of muscle protein synthesis. That may be a price they are willing to pay. But, if your hypothesis is correct, then contrary to the conventional wisdom, taking care to combine complementary protein sources in the same meal could offer a real advantage. (Here’s a quick guide to complementary protein sources.)
Doug, we hope you’ll come back on the podcast to report your results!
About the Author
Monica Reinagel is a board-certified licensed nutritionist, author, and the creator of one of iTunes’ most highly ranked health and fitness podcasts. Her advice is regularly featured on the TODAY show, Dr. Oz, NPR, and in the nation’s leading newspapers, magazines, and websites. Do you have a nutrition question? Call the Nutrition Diva listener line at 443-961-6206. Your question could be featured on the show.Â